Domiciliary monitoring of exhaled nitric oxide in the management of asthma: a pilot study

Background Whether asthma patients could benefit from home monitoring for fractional exhaled nitric oxide (flow of 50 mL/s, FeNO50) is unknown. We explore the application value of home monitoring FeNO50 in daily asthma management. Methods Twenty-two untreated, uncontrolled asthma patients were selected. Medical history, blood and sputum samples, pulmonary function, Asthma Control Test (ACT), and other clinical data of the subjects were collected. All subjects underwent daily monitoring for four weeks using a FeNO50 monitor and mobile spirometry (mSpirometry). The diurnal differences and dynamic changes were described. Compare the effect-acting time and the relative plateau of treatment between FeNO50 and mSpirometry monitoring. Results In the first two weeks, the morning median (IQR) level of FeNO50 was 44 (35, 56) ppb, which was significantly higher than the evening median level [41 (32, 53) ppb, P = 0.028]. The median (IQR) effect-acting time assessed by FeNO50 was 4 (3, 5) days, which was significantly earlier than each measure of mSpirometry (P < 0.05). FeNO50 reached the relative plateau significantly earlier than FEV1 (15 ± 2 days vs. 21 ± 3 days, P < 0.001). After treatment, the daily and weekly variation rates of FeNO50 showed a gradually decreasing trend (P < 0.05). The ACT score, sputum eosinophils, and blood eosinophils also significantly improved (P ≤ 0.01). Conclusions The daily home monitoring of FeNO50 in asthmatic patients showed significant circadian rhythm, and the sensitivity of FeNO50 in evaluating the response to treatment was higher than mSpirometry. The daily and weekly variation rates of FeNO50 change dynamically with time, which may be used to assess the condition of asthma.


Introduction
Bronchial asthma is a common chronic airway inflammatory disease that causes substantial economic and social burdens [1,2].Global Initiative for Asthma (GINA) proposes that the long-term goal of asthma treatment is to achieve symptom control and reduce the risk of acute exacerbations, irreversible airflow limitation, and treatment side effects [3].At the same time, assessment, adjustment, and monitoring form a continuous cycle in asthma treatment and management strategy.Thus, effective self-assessment and monitoring are vital for asthma patients to achieve long-term treatment goals [4].However, previous studies have shown that the natural history of asthma is heterogeneous and complex, characterized by circadian rhythms and longterm dynamic changes [5].Therefore, daily asthma management should include dynamic changes, not just the absolute value of a single measurement.
Studies have shown inconsistencies in symptoms, lung function, and airway inflammation of asthma patients [6].Self-management based on symptoms and peak expiratory flow (PEF) may still miss patients at risk of severe acute exacerbations in the future [7,8].There are also inconsistencies between physician and patient in the assessment of asthma control [9].So, perhaps more methods are needed for daily asthma assessment.Fractional exhaled nitric oxide (flow of 50 mL/s, Fe NO50 ) is one of the tools to assess airway inflammation, which is non-invasive, simple, rapid, and is currently mainly used in hospital scenarios [10].Studies have shown that increasing the number and frequency of Fe NO50 monitoring helps predict asthma control status [11].The daily fluctuation of Fe NO50 with different asthma control statuses is different [12].Studying the long-term variation pattern of Fe NO50 measurements makes it possible to identify the risk of acute exacerbations [13][14][15].Simultaneous daily monitoring of Fe NO50 can also assess compliance [10,16,17] and responsiveness to inhaled corticosteroids (ICS) for Type 2 asthma [18][19][20][21].In summary, applying Fe NO50 in daily asthma self-monitoring may be significant for asthma management.
More convenient products are applied to self-monitor chronic diseases with the development of science and technology and improved economic levels [7].The emergence of daily home monitoring devices and innovative applications has made it possible to monitor circadian rhythms and daily changes for asthma patients.However, whether asthma patients could benefit from it is not fully known due to the lack of prognostic data [22].We attempted to analyze the pattern of longitudinal dynamic changes in Fe NO50 after treatment in uncontrolled asthmatic patients who were not regularly treated.To initially explore the value of Fe NO50 domiciliary monitoring in the daily management of asthma.

Subjects
Twenty-two asthma patients who visited the respiratory outpatient of China-Japan Friendship Hospital from October 2019 to December 2021 were prospectively included.
Exclusion criteria: (1) subjects underwent other interventional clinical trials 30 days before enrollment; (2) subjects with other pulmonary diseases or other severe system diseases that may affect the conduct of the study; (3) subjects with a smoking index > 10 pack-years and a history of smoking for nearly one year; (4) subjects had been on regular asthma therapy within 12 weeks before enrollment; (5) subjects had respiratory tract infection within four weeks before enrollment.
Written informed consent was obtained from each participant.The China-Japan Friendship Hospital ethics committee approved this study (No. 2018-19-k14, approval date: February 6 th , 2018).

Study design
Enrollment stage: All subjects performed the asthma control test (ACT) [23], mini-asthma quality of life questionnaire (mini-AQLQ) [24], differential blood count, serum total IgE (enzymatic chemiluminescence, Beckmen Coulter, USA), spirometry, Fe NO50 (NIOX Vero, Circassia (Beijing) Medical Device Co., Beijing, China), differential induced sputum count.Mobile spirometry (A1, Breath Home, China) [25] and Fe NO50 monitor (NIOX Vero, Circassia (Beijing) Medical Device Co., Beijing, China) were provided to each subject to measure Fe NO50 and mSpirometry twice a day over four weeks at home.On the day of enrollment, subjects were trained on using the equipment mentioned above (viewing usage videos and on-site instruction).Subjects were contacted during subsequent use to ensure they correctly mastered the usage methods.
The treatment strategy is not affected by the study, and the patient's treatment plan is formulated by the physician, with medication recommendations based on the GINA 2018 guidelines [3].Domiciliary monitor stage: Subjects were asked to measure Fe NO50 and mSpirometry twice daily during the same period, between 06:00 to 08:00 and 20:00 to 22:00, respectively.Before medication, Fe NO50 measurement was first taken, and the results were automatically recorded on the monitor.When the subjects returned the device, all measurements were transmitted to the computer.Mobile spirometry was taken three times, and the best of the three readings were automatically uploaded to an electronic diary card.Peak expiration flow (PEF), forced expiratory volume in 1 second (FEV 1 ), forced vital capacity (FVC), the maximum expiratory flow rate at 75%/50%/25% of the vital capacity (MEF 75 , MEF 50 , and MEF 25 ) were collected by mSpirometry.
End of follow-up: ACT, mini-AQLQ, spirometry, Fe NO50 , differential blood, and induced sputum count were reviewed again.

Statistical Analysis
The normal distribution data were represented by mean ± standard deviation (sd).Non-normally distributed data were shown as median (interquartile range, IQR).A comparison of each measure before and after followup was performed using paired samples t-test and Wilcoxon signed-rank test.The categorical variables were expressed by frequency (composition ratio or percentage) and compared by the chi-square test.
Indicators and calculation formulas representing the variation of Fe NO50 and mSpirometry: diurnal variation rate = (highest in a day -lowest in a day)/ (mean of highest versus lowest in a day)×100; mean daily variation rate = mean of diurnal variation rate over 1week; weekly variation rate = (highest over two weeks -lowest over two weeks)/ (mean of highest versus lowest over two weeks)×100.
A repeated-measures analysis of variance was used to compare diurnal differences in Fe NO50 logarithmic transformed values and mSpirometry.The least-square method was used to perform curve fitting for each subject's daily monitoring results of Fe NO50 and mSpirometry, and the second derivative was used to calculate the inflection point.The inflection point in this study was the transition point for the improvement of Fe NO50 and mSpirometry, and its progress slowed down after the inflection point.The relative plateau of treatment was defined as the time to reach the inflection point [26].The effect-acting time was calculated with a Fe NO50 reduction of more than 20% and an improvement in mSpirometry of more than 10% as criteria [10,27].ANOVA and Friedman's test were used to compare the relative plateau and the effect-acting of treatment between Fe NO50 and mSpirometry.Friedman's test was also used to compare the differences in variation rates of Fe NO50 and mSpirometry.A two-tailed p-value of < 0.05 was considered significant.All statistical analyses were performed with SPSS 20 (IBM-SPSS, Armonk, NY, USA) and Matlab software (Mathworks, Inc., Natick, MA, USA).

Assessment of the disease condition before and after treatment
After four weeks of treatment, the subjects showed significant improvement in all measurements (Table 1).A significant improvement in the ACT (improvement of more than 3 points) [23] was observed in 17 subjects, with an ACT score ranging from 16 ± 4 points improved to 23 ± 2 points (P < 0.001).In mini-AQLQ, overall score, symptoms, activity, emotion, and environmental scores improved significantly (P < 0.01).The pulmonary function was significantly improved (P ≤ 0.01).In terms of inflammation, either Fe NO50 or sputum and blood eosinophils were significantly reduced (P < 0.01) (Table 1).

Diurnal variation of Fe NO50 and mSpirometry
There was a significant diurnal difference in Fe NO50 daily monitoring in the first two weeks.The morning median (IQR) level of Fe NO50 was 44 (35, 56) ppb, which was significantly higher than the evening median level [41 (32, 53) ppb, P = 0.03].However, after two weeks of treatment, the significant difference between day and night disappeared (P = 0.17).In our study, no significant differences were found in any indicators of mSpirometry between day and night (Fig. 1).

Dynamic fluctuation of Fe NO50 and mSpirometry
The best curve fit of the daily monitoring indices of Fe NO50 and mSpirometry for the subject is shown in Fig. 2. In general, Fe NO50 and Spirometry improved gradually over time with treatment.The median (IQR) effect-acting time assessed by Fe NO50 was 4 (3, 5) days, which was significantly earlier than mSpirometry (P < 0.05) (Table 2).After Bonferroni correction, Fe NO50 reached the relative plateau significantly earlier than FEV 1 (15 ± 2 days vs. 21 ± 3 days, P < 0.001), but there was no statistical difference with other indicators of mSpirometry (Table 2).

Variation rates of Fe NO50 and mSpirometry
After ICS treatment, the daily variation rates of Fe NO50 and mSpirometry showed a gradually decreasing trend.
The average daily variation rates of Fe NO50 and FEV 1 at week 1 were significantly higher than those at week 4 (P < 0.05), while there was no significant difference in PEF, MEF 75 , MEF 50 , and MEF 25 .(Fig. 3).There was a substantial reduction in weekly variation rates of Fe NO50 and mSpirometry among the participants (P < 0.05) (Fig. 4).

Discussion
Through daily domiciliary monitoring of untreated, uncontrolled asthmatic patients, we found significant diurnal differences and daily dynamic changes in Fe NO50 and mSpirometry, which can be used to evaluate asthma patients' response to treatment and condition assessment.Fe NO50 is a sensitive biomarker that reflects the inflammation of airway eosinophils [28].In the early stage of treatment, Fe NO50 was significantly higher in the morning than at night in asthmatic patients, consistent with previous findings [21,29].Although studies have shown that the decrease of FEV 1 affects the results of Fe NO50 , the effect of the change in airway diameter on Fe NO50 will be offset when the airway inflammation is higher [30,31].However, the disappearance of diurnal differences in asthmatic patients after treatment may be related to airway inflammation and lung function improvement [32,33].
The primary purpose of this study is to observe the impact of home monitoring on evaluating treatment outcomes.After four weeks of treatment, all participants showed improvement in symptoms and inflammation levels.Therefore, although different treatment plans may cause biases, we believe the impact on the conclusion is relatively tiny.
Peak-trough times of lung function and biomarkers in asthma patients may differ due to individual chronotype differences [34].Lung function fluctuates between 6-7 days in Fig. 1, but there is no significant difference.Although the measurement period was specified to avoid bias, the personal measurement time or additional measurement period was not selected according to the living habits, thus reducing the reliability of dynamic variation.No significant diurnal differences in lung function were found in this study, which may We characterized the changing trend of Fe NO50 in treated Type 2 asthmatic patients.We found that Fe NO50 could detect the treatment response for 3-5 days and reached a relative plateau for two weeks.Various indices (PEF, FEV1, MEF 75 , MEF 50 , MEF 25 ) of mSpirometry improved gradually, with treatment effect-acting time around 8~12 days and reached a relative plateau of improvement around 2-3 weeks [26,35].However, due to the relatively low goodness of fit of the fitting curves for mSpirometry in this study, the treatment turning point of mSpirometry still needs further validation with extensive sample data.
Daily monitoring found that the treatment effect assessed by Fe NO50 was significantly earlier than mSpirometry, and the time to reach the relative plateau of treatment was substantially earlier than FEV 1 .As we can see, Fe NO50 was more sensitive than mSpirometry in assessing responsiveness to asthma therapy.Meanwhile, the latest research indicates that Fe NO50 is a risk biomarker identifying patients at increased risk of lung function decline [36].
After treatment, the daily and weekly variation rates of Fe NO50 and mSpirometry showed a decreasing trend.Our study found that the average daily variation of Fe NO50 in the first week was significantly higher than in the fourth week.Therefore, the improvement of diurnal variation in Fe NO50 can also be used to evaluate the effectiveness of treatment in asthma.This variability over time suggests that the domiciliary monitoring strategy has the advantage of detecting daily and longterm changes in physiology and inflammation, providing substantial evidence to predict future exacerbations and disease assessment.After four weeks of treatment, the participants showed significant improvements in asthma control, quality of life, lung function, and inflammation.With the considerable improvement of symptoms and clinical indicators, the variation rate of domiciliary monitors gradually decreased, which is an essential clinical signal to evaluate the changes in the conditions of asthma patients.
The study had limitations.Firstly, as our study is a pilot study, all admitted patients were untreated type 2 asthma.We did not include patients with severe asthma.Still, previous studies have shown that Fe NO50 can also serve as an inflammatory marker for evaluating treatment response in severe asthma populations.[37].A large sample study was needed to demonstrate the general generalization of the variation pattern.Secondly, complex device use, frequency of daily readings, and strict monitoring times reduce the completion of measures [38,39].Still, previous studies have also shown that remote monitoring devices allow subjects to understand their self-control levels and improve patient compliance [40,41].Although there was no monitoring of drug adherence, the completion rate of this study exceeded 80%, indirectly indicating that drug adherence is still acceptable.Thirdly, our study could not compare mSpirometry A1 with the lung function laboratory device.The study found that only a portion of mSpirometry A1 (BreathHome, China) met the quality and performance evaluation standards [25].However, considering this study mainly observed dynamic changes, the research results are still acceptable.
In conclusion, this pilot study of domiciliary monitoring found that Fe NO50 in uncontrolled asthma patients has significant diurnal differences and is superior to mSpirometry in assessing sensitivity to treatment response.The dynamic fluctuation of Fe NO50 may be available for evaluating disease conditions in asthmatics.Studies with large samples and long observation periods are expected to explore mobile domiciliary monitors in asthma management.

Fig. 3
Fig. 3 Daily variation rate curves of domiciliary monitoring.a Daily variation rate curves of Fe NO50 , b Daily variation rate curves of FEV 1 , c Daily variation rate curves of PEF, d Daily variation rate curves of MEF 75 , e Daily variation rate curves of MEF 50 , f Daily variation rate curves of MEF 25 .Fe NO50 : fractional exhaled nitric oxide (flow of 50 mL/s), FEV 1 forced expiratory volume in 1 second, PEF: peak expiratory flow, MEF 75 / MEF 50 / MEF 25 : maximum expiratory flow rate at 75%/ 50%/ 25% of the vital capacity

Fig. 4
Fig. 4 Weekly variation rates of domiciliary monitoring.a Weekly variation rate of Fe NO50 , b Weekly variation rate of FEV 1 , c Weekly variation rate of PEF, d Weekly variation rate of MEF 75 , e Weekly variation rate of MEF 50 , f Weekly variation rate of MEF 25 .Fe NO50 fractional exhaled nitric oxide (flow of 50 mL/s), FEV 1 : forced expiratory volume in 1-second, PEF: peak expiratory flow, MEF 75 / MEF 50 / MEF 25 : maximum expiratory flow rate at 75%/ 50%/ 25% of the vital capacity

Table 2
Exhaled nitric oxide fraction and mobile spirometry assessment therapeutic effect † The effect-acting time was calculated with Fe NO50 reduction of more than 20% and mSpirometry improvement of more than 10% as criteria ‡ Fe NO50 vs mSpirometry (PEF, FEV 1 , MEF 75 , MEF 50 , MEF 25 ) § The relative plateau time in this study was the transition point for improvement of Fe NO50 and mSpirometry, and its progress slowed down after the transition point.Fe NO50 : fractional exhaled nitric oxide (flow of 50 mL/s), mSpirometry: mobile spirometry, PEF: peak expiratory flow, FEV 1 : forced expiratory volume in 1-second, MEF 75 / MEF 50 / MEF 25 : maximum expiratory flow rate at 75%/ 50%/ 25% of the vital capacity effect-